1. Field of the Invention
The present invention relates to a light source device suitable for wavelength division multiplexing and a wavelength control device for the light source device.
2. Description of the Related Art
An optical communication system using an optical fiber transmission line is used to transmit a relatively large amount of information. A low-loss (e.g., 0.2 dB/km) optical fiber is manufactured and used as the optical fiber transmission line. In addition, an optical amplifier for compensating for losses in the optical fiber transmission line is used to allow long-haul transmission.
As a technique for increasing a transmission capacity by a single optical fiber, wavelength division multiplexing (WDM) is known. In a system adopting WDM, a plurality of carriers having different wavelengths are individually modulated by data. Each modulated carrier provides one channel in the WDM system for transmitting optical signals. These optical signals (i.e., the modulated carriers) are wavelength division multiplexed by an optical multiplexer to obtain WDM signal light. The WDM signal light is transmitted through an optical fiber tm line. At a receiving end, the WDM signal light received is separated into individual optical signals by an optical demultiplexer. Accordingly, data can be detected according to these individual optical signals. By applying WDM in this manner, the transmission capacity by a single optical fiber can be increased according to the number of WDM channels.
In recent years, it has been desired to construct not only a system in which one point and another point are connected by an optical fiber, but also a photonic network in which a plurality of points are connected by optical fiber transmission lines. According to WDM, the broadband and high-capacity performances of an optical fiber can be effectively used. Further by using a suitable optical filter, an optical signal having an arbitrary wavelength can be selected, dropped, or added regardless of modulation method and transmission speed. Accordingly, WDM is an effective technique in constructing a photonic network. In recent years, an IP (Internet Protocol) traffic has been rapidly increased, and the introduction of WDM transmission devices for supporting this traffic is successively carried out.
In WDM, a plurality of light sources for outputting light having different wavelengths (optical frequencies). At present, a wavelength spacing of 100 GHz is mainstream, and a system for wavelength division multiplexing of about 8 to 128 channels has been developed. The light sources used in this kind of system are usually DFB (distributed feedback) lasers, and they are provided as different kinds of fixed-wavelength light sources tailored according to oscillation wavelength. For example, in a 32-channel WDM system, 32 kinds of light sources (lasers) are present. In the case that the number of WDM channels is further increased in the future, the kind of spare light sources may increase in number, causing a serious problem.
To cope with this problem, it is effective to use a tunable laser capable of covering a plurality of wavelengths. That is, the single tunable laser can support a plurality of wavelengths, so that it is effective in decreasing the kind of spare light sources for WDM. Further, in an optical ADM (add/drop multiplexer) for dropping and adding an optical signal, a tunable laser may be used in adding an optical signal, thereby allowing the optical signal to be added with a wavelength corresponding to an arbitrary idle channel.
For example, there has been developed a tunable laser having eight integrated DFB lasers whose oscillation wavelengths are controlled according to temperature. This tunable laser has an advantage such that it can output optical signals having multiple wavelengths. However, there is a problem that a wavelength control device to be used in combination with this tunable laser becomes complicated in configuration.